This project is designed to utilize a combination of approaches to elucidate the vesicular trafficking processes involved in the pathogenesis of Herrnansky-Pudlak Syndrome (HPS). The central hypothesis driving this research involves R26W, a gene that has been shown to interact with reduced pigmentation (rp) and ruby eye (ru), two mouse models of HPS, implying that R26W, rp, and ru function within the same biochemical pathway(s). Specifically, the hypothesis states that the R26 W gene and its protein product: function in vesicle trafficking pathway(s) that includes rp, ru, cappuccino (cno), pallid (pa) and muted (mu). This shared pathway(s) is critical to the biogenesis and function of three developmentally related cellular organelles (platelet dense bodies, melanosomes, and lysosomes) and is involved in the pathogenesis of HPS. There are three specific aims designed to prove the hypothesis. The first aim is to identify the specific functions of the R26W gene using targeted mutagenesis in ES cells. I will establish and characterize lines of mice carrying this targeted mutation. The second aim is to determine if R26W is a component of the BLOC-1 complex using immunoprecipitation/recapture assays. BLOC-1 is a multiprotein complex involved in organelle biogenesis and function. Its precise composition is unknown, but it includes the pa, mu, rp, and cno gene products. The third aim is to identify additional genes functioning in HPS-related pathways by isolating R26W binding ligands using two-hybrid assays and co-immunoprecipitation methods. Accomplishing the three specific aims in this project will lead to a better understanding of organelle biogenesis and the vesicle trafficking pathway(s) involved in the pathogenesis of HPS.